The present invention relates to holographic data storage media.
Many different types of data storage media have been developed to store information. Traditional media, for instance, include magnetic media, optical media, and mechanical media to name a few. Increasing data storage density is a paramount goal in the development of new or improved types of data storage media.
In traditional media, individual bits are stored as distinct mechanical, optical, or magnetic changes on the surface of the media. For this reason, medium surface area may pose physical limits on data densities.
Holographic data storage media can offer higher storage densities than traditional media. In a holographic medium, data can be stored throughout the volume of the medium rather than the medium surface. Moreover, data can be superimposed within the same medium volume through a process called shift multiplexing. For these reasons, theoretical holographic storage densities can approach tens of terabits per cubic centimeter.
In holographic data storage media, entire pages of information can be stored as optical interference patterns within a photosensitive optical material. This can be done by intersecting two coherent laser beams within the optical material. The first laser beam, called the object beam, contains the information to be stored; and the second, called the reference beam, interferes with the object beam to create an interference pattern that can be stored in the optical material as a hologram. When the stored hologram is later illuminated with only the reference beam, some of the light of the reference beam is diffracted by the holographic interference pattern. Moreover, the diffracted light creates a reconstruction of the original object beam. Thus, by illuminating a recorded hologram with the reference beam, the data encoded in the object beam can be recreated and detected by a data detector such as a camera.
The invention relates to tracking techniques for tracking holographic bit map locations on holographic media, holographic media implementing the techniques, and holographic data storage systems implementing the techniques.
In exemplary embodiments, a holographic data storage medium includes a substrate, a holographic recording material, and an optically detectable tracking pattern on the medium. The substrate may include a first substrate portion and a second substrate portion that sandwich the holographic recording material in a sandwich construction. The substrate may be formed from glass, ceramic, acrylic, a thermoplastic material such as a polycarbonate, polymethylmethacrylate (PMMA), or amorphous polyolefin, or the like. The holographic recording material may comprise a photopolymer.
The optically detectable tracking pattern may comprise an optically detectable pattern on the substrate. Alternatively, the optically detectable tracking pattern may comprise a holographic grating stored in the holographic recording material. The holographic grating, for instance, may be prerecorded in the holographic recording material. In some embodiments, the optically detectable tracking pattern may comprise a combination of one or more optically detectable patterns on the substrate and one or more holographic gratings stored in the holographic material. The optically detectable tracking pattern, for instance, may define a grating of specified physical periodicity.
If the optically detectable tracking pattern comprises an optically detectable pattern on the substrate, the pattern may be replicated, i.e., stamped, mastered, embossed, etched, ablated, or the like. The pattern may have stepped changes in the grating period or may have periodic changes in the grating period. Alternatively, the pattern may be defined by a beat frequency of at least two grating periods. Similarly, if the optically detectable tracking pattern comprises a holographic grating stored in the holographic recording material, the tracking pattern may comprise a holographic grating having stepped changes in the grating period or periodic changes in the grating period, or may be defined by a beat frequency of at least two grating periods. The holographic tracking pattern can be optically written within the holographic recording material.
In some embodiments, a holographic data storage system includes a laser that produces at least one laser beam and optical elements through which the laser beam passes. The system may further include a data encoder that encodes data in at least part of the laser beam and a holographic recording medium that stores at least one hologram. The holographic recording medium may include an optically detectable tracking pattern. The system may also include a data detector that detects the hologram, and a tracking detector that detects light diffracted by the optically detectable tracking pattern. The data encoder may be a spatial light modulator and the data detector may be a camera. The recording medium may be a disk shaped medium that is rotated relative to the other components or a card shaped medium that is translated, e.g., in x-y coordinates, relative to the other components.
The tracking detector may comprise a position sensitive detector, a segmented detector, a two-element photodetector, or the like. The holographic medium that forms part of the holographic data storage system may include one or more of the features described above.
In other embodiments, a method of determining a location on a holographic medium includes interrogating the holographic medium with light, and detecting the diffracted light. The angle of diffraction of the diffracted light may be used to indicate a position on the medium. For instance, the angle of diffraction may depend on the wavelength of light used to interrogate the medium, and on the period of the optically sampled grating. As such, the measured diffraction angle may be used to indicate particular positions on the medium. The medium may be a disk-shaped holographic medium or a card shaped holographic medium. Interrogating the holographic medium with light may comprise interrogating the disk with a probe beam, or alternatively may comprise interrogating the disk with the holographic reference beam. In either case, the beam may move radially or tangentially across the disk. In this manner, a track location defined by a particular diffraction angle of the diffracted light can be located with precision.
Additional details of these and other embodiments are set forth in the accompanying drawings and the description below. Other features, objects and advantages will become apparent from the description and drawings, and from the claims.